1. Field of the Invention
The present invention relates to the purification of aqueous alkali metal chloride solutions by removal of ammonium and iodine values therefrom, and particularly wherein, simultaneously, the ammonium values are consumed by oxidation to molecular nitrogen, followed by a degassing treatment.
2. Description of the Prior Art
It is known to this art to electrolyze aqueous solutions of sodium chloride to produce chlorine.
When electrolysis is carried out according to the so-called xe2x80x9cmembranexe2x80x9d technique, it is necessary to carefully purify the starting aqueous solution to remove the normal impurities in sodium chloride, such as calcium, magnesium and sulfates. For example, precipitations by means of sodium carbonate, then absorptions onto resins are employed for such purpose; such methods are described in Ullmann""s Encyclopedia Of Industrial Chemistry, Vol. A6, 448 (1986). Depending upon its particular origins, the starting sodium chloride solution can also contain iodine in the iodide state I-.
U.S. Pat. No. 4,483,754 relates that the presence of iodine in an aqueous sodium chloride solution in a membrane technique leads to a rapid degradation of the membranes. According to this ""754 patent, the iodine present in the aqueous solution in the iodide form (oxidation state xe2x88x921) is oxidized to the molecular iodine form (oxidation state 0), then such molecular iodine is removed from the aqueous chloride solution either by absorption onto active charcoal or onto an anionic exchange resin, or by stripping with air in a column. The examples reflect that an aqueous sodium chloride solution containing 0.8 ppm (Example 1) and 0.7 ppm (Example 3), expressed as iodine, can thus be obtained. Thus, such purification to remove iodine values is considered to be sufficient.
EP-0,399,588 describes a technique for purification of an aqueous sodium chloride solution in which iodides are oxidized to molecular iodine which is adsorbed on an ion-exchange resin and then, in a downstream stage, ammonium ions are oxidized to molecular nitrogen which is removed from the solution by stripping with air.
An aqueous sodium chloride solution containing from 0.2 to 0.5 ppm of iodine is thus obtained. It is indeed described in column 2, lines 24-26, that it is advisable to avoid an excessive oxidation resulting in the formation of IO3xe2x88x92 anions (oxidation state +5).
Published application IRE No. 30,732, dated November 1989, relates that iodine contained in aqueous sodium chloride solutions, in the iodide form, is oxidized to periodate in the cell over the course of electrolysis. This periodate precipitates in the membrane and damages it. According to this ""732 publication, barium is added to the solution, prior to electrolysis; there is thus formed a very fine, highly insoluble precipitate of barium periodate.
It has now been determined that the above solution is not satisfactory when the amount of iodine is significant, because the precipitate has a tendency to accumulate in the membrane and reduces its performance. It is thus necessary, for the technique of membrane electrolysis, to reduce as much as possible the amount of iodine. The aforesaid prior art relating to the elimination of iodine must employ resins therefor.
It has now been found that it is far simpler and more effective to oxidize the iodine to periodate and then to remove such periodate by filtration. An aqueous sodium chloride solution containing at most 0.1 ppm of iodine is thus obtained which can be electrolyzed without damaging the membranes of an electrolytic cell.
Briefly, the present invention features a technique for the purification of an aqueous alkali metal chloride solution containing iodine, comprising oxidizing such iodine values to the oxidation state +7, namely, the periodate form, and then removing the periodate.
More particularly according to the present invention, the oxidation of iodine, namely, of iodide, of iodine or of iodate, is advantageously carried out at any pH with active chlorine. To oxidize by mean of active chlorine, it is sufficient to inject chlorine, chlorine water or hypochlorite, such as household bleach, into the starting solution. It is possible to monitor the oxidation by a measurement of the oxidation/reduction potential. The amount of oxidizing agent must be at least slightly greater than the stoichiometric amount in order to oxidize the iodide to periodate. Indeed, a large excess in relation to this stoichiometry is preferable in order to oxidize other compounds present in the brine and exhibiting a reducing power with regard to the oxidizing agent. This oxidation can be carried out at room temperature, but in order to provide a more rapid reaction, it is preferable to operate at a temperature of at least at 60xc2x0 C.
The kinetics of oxidation of iodine to periodate are rapid. It is preferable to oxidize at more than 40xc2x0 C. to increase the speed of formation of a precipitate of iodine in the +7 oxidation state. At 60xc2x0 C., the desired precipitate is formed after a few minutes. Preferably, from 20 to 90 min are required for proper efficiency.
According to the present invention, iodine is oxidized to the IO4xe2x88x92 ion form, i.e., the periodate form. To provide the most complete possible precipitation of the periodate, it is preferable for the reaction to be carried out in a basic medium and preferably at a pH of from 11 to 13. The pH can be adjusted by a basic agent such as sodium hydroxide. The addition of the oxidizing agent and the basic agent can be conducted in any order.
It is preferable that the starting aqueous alkali metal chloride solution should contain at least one barium, calcium, strontium or magnesium cation in order to form the periodate. The treatment for purification from iodine is thus carried out on the crude aqueous alkali metal chloride solution, namely, prior to removal of the calcium, magnesium, etc., according to conventional purifications described in the aforesaid Ullmann text. The purification of the invention can also be carried out after the conventional purifications from calcium or magnesium, provided that these treatments have been conducted using barium carbonate. Barium cations thus remain in the aqueous alkali metal chloride solution. The removal of the periodate can be effected by a simple filtration, a decantation, or any equivalent means or any combination thereof.
After the treatment of the invention, and if the aqueous alkali metal chloride solution has to be transported over resins or any material sensitive to active chlorine, it is advisable to destroy the possible excess of oxidizing agent. Such destruction or consumption of active agent is per se known to this art For example, SO2 can be used therefor.
The present invention also features a method in which ammonium is removed in addition to iodine. Thus, the starting aqueous solution may also contain ammonium cations. It is desirable to remove dissolved ammonium to avoid the formation of NCl3, an unstable compound which can decompose violently. The ammonium values are oxidized to molecular nitrogen and then the nitrogen is removed by degassing by placing the solution under vacuum, or by stripping it in a column with an inert gas or with air. This oxidation is also per se known to this art, and is carried out at a pH less than 10 and preferably ranging from 6 to 10. In a preferred embodiment of the invention, the oxidation of the ammonium and iodine is carried out simultaneously. It suffices that this be under pH conditions for the oxidation of ammonium since these are the most limiting.
It is advantageous to use a single oxidizing agent for the iodine and ammonium values. Preferably, active chlorine is employed therefor.
The amount of active chlorine used to destroy/consume the ammonium is advantageously greater than the stoichiometric amount required and preferably ranges from 1.5 to 2.5 times the stoichiometric amount. Downstream, the nitrogen is degassed and then the pH is adjusted by addition of a basic agent, for example sodium hydroxide, to effect the precipitation of periodate; the operation is then conducted as described above.
In order to further illustrate the present invention and the advantages thereof, the following specific examples are given, it being understood that same are intended only as illustrative and in nowise limitative.